Research On Precoding In Cognitive MIMO System

The cognitive MIMO technology has both the advantages of cognitive radio and MIMOtechnology, becoming the novel technology to overcome the contradiction between frequencyspectrum resource supply and demand. The key problem in the MIMO technology system is howto handle the co-channel interference between congtive users (CUs) and primary users (PUs).This paper takes the precoding technology as the cut-in point and uses the subspace theory,realizes space multiplying of CUs and PUs to enhancing the CU’s transform speed and reducingthe PUs interference. The main contents of the paper are as follows:1. Analyzing influences to the channel capability caused by the co-channel interferencewhen the frequency spectrum resource is shared among CUs and PUs.The necessity andfeasibility of using precoding to eliminate interference was pointed out. Against the thecharacteristics of cognitive MIMO system such as double systems, authoration of spectrumexploitage and definite orders of users, co-optimization of interference and transmiting power,contrainst of CUs and PUs’ spectrum authority and character abstracting of CUs’transmition wasput forward, making the technology approach of this paper clear.2. Aimed at the key problem of co-channnel interference when the frequency spectrumresource is shared among CUs and PUs, the idea of gradational processing was introduced andthus the two-degree precoding algorithm based on subspace orthogonality was proposed. Thisalgorithm converts users’ spectrum authority contrainst to priority of interference controlhandling.To the CUs with high spectrum authority, the subspace orthogonal projection was firstlyused to make the PUs and CUs transform space be orthogonal and avoid the interference to PUs.To the CUs with the same priority, design the optimal precoding method aiming at maximizingthe SNR to reduce the co-channel interference and enhance the transmition rate as more aspossible. The simulation results indicated that the proposed algorithm efficiently reduced theinterference and improved the system performance and it is worth to be pointed out that thegradational frame constructed by this algorithm converts combined process of double systems toserial process of single system and can tolerate mature technology of traditional MIMO systemand cognitive radio system.3. In both MIMO system and cognitive MIMO system, there is a technology bottleneck ofhigh process complexity in precoding method. A low complexity precdoing algorithm based onsubspace tracking was proposed to solve this problem which uses the strong correlation amongneighbor symbols in slow fading channel. This algorithm uses co-users power leak age describedby Forbenius norm as optimal target, decides the zero forcing precoding matrix as the filtercoefficient and introduces the LMS adaptive filter structure to realize orthogonal subspacetracking according to the recursive relation of filter coefficient shown by deduction of the matrixinstantaneous gradient. The simulation results showed that convergence rate was faster, trackingability was stronger and computation complexity was lower in slow fading channel, comparingwith traditional CR-BD algorithm. 4. Aimed at the problem of CUs transimition character abstracting, the joint transceiverdesign precoding algorithm based on subspace decomposition was proposed.The MIMOprocessing was applied to traditional detect-avoid transmition mechanism, dividing into channeldetection stage and data processing stage. The covariance matrix of received date was processedby subspace decomposition and MDL estimation during the channel detection, in which the noisespace and equivalent channel information of PUs’ interferencing channel was acquired. Thesimulation result showed that the proposed algorithm without the knowledge of interferencechannel’s CSI can improve detect performance to reduce interference between users byincreasing snapshots and raising receiving SNR, thus achieve system error symbol rateperformance similar to existing CSI precoding algorithm.